Many optical energy applications require high intensity, spatially uniform, white light that does not significantly heat the surrounding environment in the near field and/or far field. More specifically, many applications require correlated color temperatures between 4100–4900K (i.e., white light) with a color rendering index (“CRI”) between 90 to 100.
Correlated color temperature (“CCT”) is a numerical assignment of the apparent color of a light source (i.e., as viewed by the human visual system) and is measured in degrees Kelvin. Color rendering is how well a light source renders color (i.e., in the course of interacting with an object) as compared to how well daylight renders color (i.e., in the course of interacting with the same object).
Traditional light sources, however, suffer from, for example, but not limited to, combinations of a poor CRI, poor CCT, poor intensity, short usage life, large power electrical consumption, large package size, thermal energy, and/or are electrically and/or optically inefficient.
Tungsten filament lamps, for example, while providing high intensity optical energy with high CRI values, emit optical energy that has a poor CCT (i.e., about 3000K, which correlates to the color yellow) for white light applications. In addition, tungsten filament lamps have a low electrical to optical efficiency and, thus, require large amounts of electrical power to generate high intensity optical energy, which results in large quantities of thermal energy. Furthermore, high power tungsten lamps have a low lamp lifetime, usually operating for about 500 hours.
Tungsten-halogen lamps, when used in conjunction with filters, produce a CCT of above 4000K but still suffer from many of the same disadvantages of Tungsten filament lamps.
Metal halide lamps have a high luminous efficiency (“electric energy” to “optical energy” efficiency) and produce optical energy with a CCT of around 5000K (bluish white), which is just above the white light range. However, Metal halide lamps also emit optical energy below and above the human visual system. The optical energy above the white light CCT range is referred to as infrared light. Infrared light optical energy is sensed as thermal energy or heat. The optical energy below the white light CCT range is referred to as ultra violet light and, in many circumstances an unwanted or damaging byproduct. Xenon arc lamps provide optical energy with higher intensity than metal halide lamps, but have a low luminous efficiency and low lamp life time (around 500 hours). Furthermore, traditional light sources such as arc lamps, for example, when used as a light source for a less than spherical illumination region, are optically inefficient. The full spherical discharge of optical energy is difficult to capture into a particular illumination region.
A light emitting diode (“LED”) emits optical energy over specific CCT's within the white light CCT range. However, commercially available LED's that emit white light have low CCT and have poor control. In addition, LED's provide insufficient optical energy for most illumination applications.
An improved optical system is needed.